The invention relates to a method for producing a dispersion of nanoparticles having an essentially oxide-free surface in a liquid phase, a two-phase system being used. The invention relates furthermore to the dispersion produced with this method.
For liquid-processed electronic components based on the use of dispersed nanoparticles (“inks”), it is necessary that these particles have a defined and known surface quality. Thus, for example the presence of a surface oxide layer on a metallic or semiconducting nanoparticle significantly increases the electrical resistance of the normally conducting or semiconducting thin films produced therefrom in electronic components and reduces the efficiency of these components or prevents their function entirely. Likewise, subsequent processes of the films, such as e.g. sintering, are impeded by the oxide shell. As a result of the high specific surface, degradation of the surface in air or in water is effected too rapidly to be able to further process the particles produced initially with a defined surface which is free of disruptive atoms without changing the surface.
Various methods for resolving this problem previously have now become known from the state of the art.
Thus US 2008/0063855 A1 describes storage of nanoparticles under an inert gas atmosphere, such as e.g. nitrogen, argon or the like, in a closed environment, such as e.g. a glove box. With this method, it is however disadvantageous that high expenditure on equipment is required for this purpose and, due to contamination of the inert gas atmosphere with oxygen or water, a susceptibility to faults cannot be avoided. In the case of the method described in the above-mentioned US patent, it is also disadvantageous that this cannot be achieved economically.
In WO 2009/125370 A1, an in situ reduction of oxidised particles during production is described. One possibility for this is grinding of bulk material. By adding a liquid reducing agent during grinding or alternatively by a suitable chemical property of the mill alloy, the particles are freed of oxide directly during their production or the formation of oxide is suppressed. However, it is unfavourable with this described process that the structural properties (crystallinity, size distribution) of the ground particles is greatly non-homogeneous, these then in turn having a disadvantageous effect on the film formation or printability and also the electrical properties resulting therefrom.
Furthermore, a method is known from EP 2 067 743 A1, in which an organic functionalisation of an oxide-free particle surface is undertaken by bonding a monolayer of organic molecules covalently to this surface. It must be mentioned as a disadvantage in this method that the organic monolayer impairs the electrical properties of the thin films produced from the particles and that it is also not ensured that the oxidation is necessarily prevented.
Finally, etching of the surface oxide of silicon nanoparticles by means of an acid in dispersion of a polar solvent or also alternatively by etching the particle shells in the already deposited film after evaporation of the solvent is known from DE 10 2006 024 490 A1. It is disadvantageous with this method that the process demands precise control of the etching rate by means of the concentration of the acid and also etching duration in order to avoid over- or under-etching. In the case of etching in dispersion, the process is restricted to polar solvents, such as e.g. water or alcohols, in order to dissolve the acid. Disadvantages result therefrom for the stability of the dispersion (potential segregation, reoxidation in the case of too low an acid concentration, restriction for printing process; preference for organic solvents which are easy to evaporate and less polar). Since the acid is still present on the deposited film during processing, a potential contamination source or a risk with respect to operating safety is hence provided.